The frontal surface area of an antenna mounted on an aircraft, under a radome, is of critical importance with respect to the aerodynamics of the aircraft. This is because of the drag created by the radome and the resulting effects on aircraft performance and fuel consumption. With reflector antennas that must be rotated about their azimuthal axes, the “swept arc” of the antenna is larger than the overall width of the main reflector of the antenna. This necessitates a commensurately wide radome, thus increasing the frontal surface area of the radome and consequently increasing the drag on the aircraft.
Referring to FIG. 1, the diameter of a swept arc “A” of a main reflector of a prior art antenna system can be seen when the azimuthal axis of rotation is located rearwardly, or behind, an axial center of the main reflector, as is conventional with present day reflector antenna systems. The outermost edges of the main reflector are also noted. This diameter is noted by dimension “B”. The diameter of the swept arc produced by the main reflector is considerably larger than the diameter of the main reflector itself when the azimuthal axis of rotation is located at, or rearwardly of, the center of the main reflector.
It is therefore extremely important that the height and width (i.e. depth) of a reflector antenna be held to the minimum dimensions consistent with the required electromagnetic performance of the antenna. More particularly, it is important for the main reflector of an antenna intended to be mounted on an outer surface of an aircraft, to be mounted in such a manner that the swept arc of the antenna is minimized when the antenna is rotated about its azimuthal axis. Minimizing the swept arc of the antenna would thus minimize the dimensions of the radome required to cover the antenna, and thereby minimize the corresponding drag created by the radome while an aircraft on which the radome is mounted is in flight.
Still another consideration in minimizing the swept arc is the physical length of the feed horn mounted at the axial center of the reflector (i.e., at the vertex). To maximize antenna performance, in some instances it would be desirable to use a longer feed horn on the reflector. However, using the longer than typical length feed horn necessitates increasing the depth of the reflector itself. Increasing the overall depth of the reflector means increasing its overall diameter or aperture size, and thus increasing its swept arc. Thus, there exists a need for a reflector antenna design that allows the use of an elongated feed horn which can be integrated into the reflector of the antenna without requiring an increase in the depth and the overall aperture size of the antenna.